Murphy is out there, ready to make your life miserable. Whether because of bad weather, carrier problems, or just plain bad luck, an ISP's connection to a NAP, another provider, or another POP will become unavailable at some point, potentially resulting in the inability to reach all or a set of destinations. A redundant network enables traffic to utilize an alternative path to reach those destinations until the problem has been corrected. A well-designed ISP network has POPs connected to multiple NAPs, other provider networks, and multiple other POPs, as illustrated in Figure 2-5.

Figure 2-5. A Redundant Network Provides More Reliable Connectivity
It's important to understand that peering and interconnection redundancy to other networks are usually provided on a global basis. In other words, if a connection to a provider becomes unavailable via the primary traffic exchange point, the next closest exchange point will be selected. The idea behind this is to not provision redundant capacity from the same location to another network, but to ensure that enough spare interconnection and backbone capacity exists to accommodate failures in one (or more) locations in the network. With this approach, provisioning more interconnection and NAP circuits in more geographically optimal locations can offset costs of the redundant connections, benefiting the network during both normal operation and failure scenarios by providing this redundancy on a global versus POP-by-POP basis. Figure 2-6 illustrates a less-than-optimal connectivity model, and 2-7 illustrates a redundant interconnection model.

Figure 2-6. Less-Than-Optimal Connectivity Model
Figure 2-7. Redundant Interconnection Model
A provider's sparing plan should also be considered when discussing redundancy. Most providers keep an on-site supply of critical hardware components and manage spare equipment as a ratio of working:spare. The number of spare components usually depends on the critical nature of the component, as well as the component's theoretical 4848MTBF (Mean Time Between Failures). Some providers choose to outsource the sparing services, normally to vendors who maintain geographically dispersed depots and share the inventory among several customers. Although this approach does potentially increase MTTR (Mean Time To Repair) when problems occur